The invention concerns a plasma reactor for processing a semiconductor wafer or workpiece and in particular for etching high aspect ratio openings through a thin film structure on the wafer. In such processes, the higher the aspect ratio of the opening to be etched (i.e., the smaller the opening diameter and the deeper the opening depth), the greater the etch selectivity that is required. Typically, the opening is defined by a photoresist layer, and the etch selectivity is a measure of the etch rate of the thin film material to be etched versus the etch rate of the photoresist. There are two seemingly conflicting needs for such a process. One need is to improve the overall etch rate of the process to increase throughput or productivity. The other need is to improve selectivity in order to be able to form higher aspect ratio openings (smaller diameter, deeper depth) in order to fabricate smaller or finer semiconductor structures. That these two needs apparently conflict may be seen from the following. Processes that exhibit high etch selectivity (and are therefore capable of etching high aspect ratio openings) typically involve capacitively coupled plasmas. Unfortunately, such plasmas are incapable of providing a high ion density and therefore exhibit a low etch rate. On the other hand, processes that exhibit a high ion density (and are therefore capable of etching at high etch rates) typically involve inductively coupled plasmas. Unfortunately, such plasmas tend to form the most highly reactive species at higher power level or higher plasma density and therefore are incapable of achieving high etch selectivity. This problem worsens as the plasma source power and ion density increases.
Such problems are particularly acute when etching silicon dioxide films or oxygen-containing films that overlie silicon or non-oxygen containing films, when using fluorine as the principal etchant, for example. Fluorine etches silicon faster than silicon dioxide, and so special measures must be taken in order to etch a silicon dioxide film without etching an underlying silicon film. Such special measures involve the use of fluoro-carbon or fluoro-hydrocarbon process gases. The gases dissociate in the plasma to form fluorine-containing etchant species and carbon-containing polymer precursor species. The polymer precursor species accumulate as a protective coating on any exposed silicon surfaces but not on silicon dioxide surfaces, thus enhancing etch selectivity. The low selectivity of typical inductively coupled plasmas is attributable to the tendency of such plasmas to at least nearly completely dissociate the process gases into their most fundamental components, such as free fluorine. Free fluorine is highly reactive and tends to etch both silicon dioxide as well as non-oxygen containing materials (such as polymer and photoresist), so that little or no etch selectivity is possible with such an etchant. In contrast, the high etch selectivity of capacitively coupled plasmas is due to their tendency to effect only limited dissociation of the process gases (thus producing very little or no free fluorine), so that the etchant species are more complex fluoro-carbon species that tend not to attack polymer and photoresist.
The dissociation behavior of inductively coupled plasmas and capacitively coupled plasmas is experimentally confirmed using optical emission spectroscopy (OES) measurements. Such OES measurements taken in capacitively coupled plasmas indicate a large population of complex fluorocarbon species, in other words ions or radicals consisting of a number of carbon and fluorine atoms, and a small population of simple fluorocarbon species or free fluorine and free carbon. OES measurements taken in inductively coupled plasmas indicate an opposite distribution of species, namely a large population of simple fluorocarbon species and free fluorine and a smaller population of the more desirable complex fluorocarbon species. This phenomenon is associated with undesirable elevations in the electron energy distribution high energy tail.
An inductively coupled plasma is maintained using an antenna having a large inductance, such as a coil antenna. Various types of antennas for maintaining capacitively coupled plasmas are known, each of which has a large capacitance, and these typically consist of a pair of electrodes. Usually, one electrode is the wafer support pedestal and the other electrode is the overhead chamber ceiling, the RF source power being connected across the two electrodes. One type of overhead electrode is disclosed in U.S. Pat. No. 5,900,699 and consists of a set of conductive spokes extending radially outwardly from the center of a circle and another set of spokes, electrically separate from the first set, extending radially inwardly from the circumference of the circle, the two sets of spokes being interleaved. Typically, the length and spacing of the spokes, which is limited by the size of the reactor, limits antenna resonance to the UHF region. Preferably, the antenna is driven at a UHF frequency that is the resonant frequency of the antenna. At resonance, RF currents in adjacent spokes are at different phases, while the phase difference between the inner and outer arrays of spokes is about 180 degrees. The problem with such an antenna is that the impedance match with the UHF power source is relatively unstable over fluctuations in the plasma, either in the match or in the current distributions in the individual spoke legs, the antenna/plasma combination being highly sensitive to plasma load/mode fluctuations. This problem has been addressed by imposing a large capacitance in the form of an insulating layer between the spoke antenna and the plasma. The larger this additional capacitance, fluctuations in plasma load impedance have proportionately less effect on the overall load impedance presented to the UHF signal generator. However, such a large capacitance between the antenna and the plasma acts as a significant voltage divider and does not provide optimum efficiency. Moreover, such an antenna is expensive because it requires a UHF signal generator. Thus, with such unstable impedance match characteristics and high cost, the spoke antenna has not seemed to be a practical choice.
Capacitively coupled plasmas typically are formed at relatively high chamber pressure and low plasma RF source power. The plasma RF source power is coupled into the chamber by applying it across the wafer support pedestal and the overhead ceiling. In such a reactor, the RF power controls both the plasma ion density and the electron temperature at the wafer surface. Inductively coupled plasmas typically are formed at relatively low chamber pressure and high plasma RF source power. The plasma RF source power is coupled into the chamber by a coil antenna near the ceiling or side wall of the chamber whose power level determines ion density. (Electron temperature at the wafer surface can be controlled in such a reactor independently of ion density by a separate RF power source connected across the wafer pedestal and the overhead ceiling.) Thus, whether a plasma is maintained by inductive coupling or by capacitive coupling depends upon the type of antenna employed as well as upon the conditions within the chamber, particularly chamber pressure. Typically, a plasma either is inductively coupled and therefore has the potential for high ion density and etch rate but low etch selectivity or is capacitively coupled and has high etch selectivity but low ion density and etch rate. Thus, there has seemed to be no way to realize the high selectivity of a capacitively coupled plasma without foregoing the high etch rate of an inductively coupled plasma.
A plasma reactor for processing a semiconductor workpiece, includes a reactor chamber having an insulating chamber ceiling and side wall and a workpiece support pedestal within the chamber, a process gas conduit and gas distribution orifices facing the interior of the chamber and coupled to the process gas conduit. A conductive RF enclosure outside of the chamber overlies the insulating ceiling and has a conductive side wall with a bottom edge supported on the chamber ceiling and a conductive RF enclosure ceiling supported on a top edge of the conductive side wall, and a conductive post extending parallel with the conductive side wall from a center portion of the RF enclosure ceiling toward the chamber ceiling.
An RF power applicator of the reactor includes inner and outer conductive radial spokes. The set of inner conductive spokes extends radially outwardly from and is electrically connected to the conductive post toward the conductive RF enclosure side wall. The set of outer conductive spokes extends radially inwardly toward the conductive post and is electrically connected to the conductive side wall. In this way, the inner and outer sets of conductive spokes are electrically connected together, the combination of the inner and outer set of spokes with the conductive enclosure having a fundamental resonant frequency inversely proportional to the height of the conductive enclosure and the lengths of the inner and outer set of conductive spokes. An RF source power generator is coupled across the RF power applicator and has an RF frequency corresponding to the fundamental resonant frequency.
In one embodiment, the RF power source is connected across an RF tap point on one of the spokes and the conductive enclosure and the RF tap point is near the end of one of the inner set of spokes. In this embodiment, the inner set of spokes are interleaved with the outer set of spokes and are generally coplanar. In this embodiment, the inner and outer sets of spokes are in a plane near the bottom edge of the conductive side wall of the conductive enclosure.
The reactor may further include an insulating base between the RF power applicator and the chamber ceiling. The insulating base has a capacitance which brings a load impedance presented to the RF power source closer to the output impedance of the RF power source than it would otherwise be without the insulating base. The gas distribution manifold and the gas distribution orifices may be formed in the insulating base, the orifices opening out in a bottom surface of the insulating base toward the workpiece support pedestal.
The insulating base may have a top portion defining a 3-dimensionally shaped surface, the inner and outer spokes of the RF power applicator conforming to the 3-dimensionally shaped surface. The spokes of the RF power applicator are shaped by the 3-dimensionally shaped surface so as to render a radial distribution of plasma ion density more uniform. The 3-dimensionally shaped surface may correspond to a multi-radius dome.
The three dimensional shape can modify the power distribution to the plasma. This can add uniformity to the plasma ion density, but more generally, it can tune the process uniformity such that it is not necessarily more or less uniform, but just more beneficial for the process. For example, MERIE chambers intentionally induce a center strong plasma without magnetic fields, and then make the process uniform with their magnetic fields. If this source were to be included with MERIE style magnets, this 3-D surface could be used to enhance this center strong profile, whether or not it makes the source generation of the ion density more uniform.
In one embodiment, the fundamental resonant frequency is a VHF frequency and the RF currents in all of the spokes of the RF power applicator are in phase at the fundamental resonant frequency.